Code review comment for lp:~aacid/mir/fix_forward_declarations

I think this attempt is misguided and that you should use -Wno-mismatched-tags in qtmir.

/1/ According to the language standard there are no differences between "struct" and "class" in this (declaration) context. The only differences are in the context of definitions (of which there will only be one. So the warning is pointless.

/2/ We did try to keep clang happy for a while on Mir, but eventually ran into some standard library types that were declared "struct" in some headers and "class" in others. So problems existed outside code that we control.

So there's no advantage to using this warning, and a definite cost.